1. Field of the Invention
The present invention relates to a liquid crystal display to be used as a flat panel display or the like and a method of manufacturing the same.
2. Description of the Related Art
An active matrix type liquid crystal display has a lot of picture elements and good image quality and is therefore expected to be promising as a flat panel display. However, as the number of picture elements is increased and a display area is enlarged, it becomes difficult to manufacture the entire surface of an active matrix substrate without any defect. For example, a 640.times.480-pixel color display actually has 640.times.3.times.480=921,600 picture elements, three picture elements for each pixel of the color display. Therefore, failure factors of the display such as a disconnection or a short-circuit must be eliminated throughout all of about 920,000 transistors, 1,920 signal lines, and 480 scanning lines.
FIG. 7 is a figure of equivalent circuit showing a circuit arrangement of a conventional active matrix substrate. As shown in FIG. 7, this unit picture element is constituted by one transistor and one auxiliary capacitor, and this arrangement is functionally satisfactory.
Possible display defects, however, are a "line defect" and a "point defect", and the "point defect" is far more easily generated. This is because the number of picture elements is much larger than that of lines and therefore a defect probability of the point defect differs from that of the line defect. Several modes are considered in the point defect. In addition, the point defect is easily generated due to a failure of a transistor and a short-circuit in an auxiliary capacitor. In the circuit shown in FIG. 7, if the transistor or the auxiliary capacitor fails, a corresponding picture element fails, and this bad element becomes a point defect.
Several methods of eliminating the point defect are known. FIG. 8 shows a conventional picture element as an example of these methods. In this figure of equivalent circuit of the picture element, two transistors as active elements are used in a single unit picture element. That is, as shown in FIG. 8, one transistor (Tr1) is connected to an nth scanning line and an mth signal line, and the other transistor (Tr2) is connected to an (n+1)th scanning line Gn+1 and the mth signal line Sm. If both the transistors Tr1 and Tr2 operate normally, a picture element voltage has a value written via the transistor Tr2. If the transistor Tr1 fails, the transistor Tr2 must be cut. In the latter case, however, a signal to be written by the transistor Tr1 is no longer a normal signal but becomes the same signal as that of a picture element immediately above the transistor Tr1. Note that when an image does not largely change upon shifting of one picture element as in a display such as a television image, this defect is not remarkable.
In an image apparatus for OA equipment, however, information cannot be correctly displayed unless each and every dot on a screen correctly indicates a signal. If two transistors are connected to a scanning line of this point defect portion in order to prevent this inconvenience, the capacitance of the scanning line is increased and propagation of a gate pulse is delayed. As a result, the sizes of the screen and the picture element are increased.
In order to reliably transmit a normal signal to a defective picture element, a plurality of scanning lines or signal lines may be provided for each picture element. In this method, however, the area of a wiring region is increased to reduce an aperture ratio, and adjacent wirings easily form short-circuits. That is, new defect generating factors are introduced in picture elements.
As described above, in the active matrix type liquid crystal display or the like, recovery methods based on conventional schemes are not effective to prevent generation of the point defect caused by, e.g., a failure of a transistor or a short-circuit in an auxiliary capacitor. In addition, if a redundant circuit is added to the display, a disadvantage such as an increase in manufacturing load or a reduction in aperture ratio is apparently caused. These disadvantages are pronounced when the size of an image apparatus is increased, its precision is improved, and its resolution is increased.